C. reinhardtii sheds its flagella in response to acidification. Previously, we showed correlations between pH shock, deflagellation, and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] production, but 100% of cells deflagellated by 5 s, which was the earliest that Ins(1,4,5)P3 accumulation could be accurately measured by techniques available to us at that time (Quarmby, L. M., Y. G. Yueh, J. L. Cheshire, L. R. Keller, W. J. Snell, and R. C. Crain. J. Cell Biol. 1992. 116:737-744). To learn about the causal relationship between Ins(1,4,5)P3 accumulation and deflagellation, we extended these studies to early times using a continuous-flow rapid-quench device. Within 1 s of acidification to pH 4.3-4.5, 100% of cells deflagellated. A transient peak of Ins(1,4,5)P3 was observed 250-350 ms after pH shock, preceding deflagellation. Preincubation with 10 microM neomycin, which prevents hydrolysis of phosphatidylinositol 4,5-bisphosphate, inhibited both the transient production of Ins(1,4,5)P3 and the subsequent deflagellation. The nonspecific Ca2+ channel blockers La3+ and Cd2+ prevented flagellar excision induced by mastoparan without inhibiting rapid Ins(1,4,5)P3 production. Likewise, the Ins(1,4,5)P3-gated channel inhibitors ruthenium red and heparin blocked deflagellation in response to mastoparan. These studies were extended to mutants defective in flagellar excision. Fa-1, a mutant defective in flagellar structure, produced Ins(1,4,5)P3 but failed to deflagellate. These results support a model in which acid pH activates a putative cellular receptor leading to G-protein dependent activation of phospholipase C and accumulation of Ins(1,4,5)P3. These events are upstream of Ins(1,4,5)P3-dependent Ca2+ entry from the medium, and of deflagellation.

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